1. Field of the Invention
The present invention relates to gas turbine engines and, more particularly, to an air/fuel mixer for a combustor. The type of gas turbine engine may be used in power plant applications.
2. Description of the Prior Art
Low NOx emissions from a turbine engine of below 10 volume parts per million (ppmv) are becoming an important criterion in the selection of turbine engines for power plant or aircraft applications. The current technology for achieving low NOx emissions involves a combination of a combustor with a fuel/air premixer. This technology is known as Dry-Low-Emissions (DLE) and offers the best prospect for clean emissions combined with high engine efficiency. The technology relies on a higher air content in the fuel/air mixture.
An air/fuel mixer is described in copending U.S. patent application Ser. No. 09/742,009, filed on Dec. 22, 2000, and assigned to the present applicant, which is herewith incorporated by reference. As described in that patent application, it is important to provide a uniform fuel/air mixture in the burn zone of a combustion chamber. The challenge is to achieve low emissions over different load conditions, yet obtain low cost of operation.
Although the above-mentioned application describes a particular fuel manifold assembly for a DLE system, it does not teach the environment in which the assembly would be used in a combustion chamber. For one thing, the burn zone should be located in a location within the chamber where the flame can be stabilized and to avoid coming into contact with the walls of the combustor can forming the chamber. It is also important to prevent cooling air from entering the burn zone formed in the combustion chamber.
It is an aim of the present invention to provide an improved fuel/air mix in a burn zone formed within the combustion chamber.
It is a further embodiment of the present invention to provide an air/fuel mixer using a fuel manifold instead of a nozzle.
It is a further aim of the present invention to provide a combustion chamber with a low power ignition stage and a second stage for full load combustion.
A combustion system in accordance with the present invention comprises a gas turbine engine having an annular cylindrical combustion casing with an inner wall and a radially spaced outer wall defining a combustion chamber, an annular air/fuel inlet at an end of the combustion casing, concentric with the inner and outer walls, a combustion chamber outlet downstream of the combustion chamber, the air/fuel inlet including a diffuser passageway formed between diffuser portions of the inner and outer walls respectively wherein each inner and outer diffuser wall portion has an upstream and a downstream portion relative to the air flow; the diffuser passageway formed by the adjacent inner and outer diffuser wall portions includes a converging cross-sectional section at the upstream portion of the inner and outer diffuser wall portions and a diverging cross-section at the downstream portion of the diffuser inner and outer wall portions and a throat is defined at the narrowest part of the passageway formed by the diffuser inner and outer wall portions; a concentric fuel manifold ring is provided upstream of the diffuser passageway whereby the manifold ring is located in axial alignment upstream of the diffuser passageway whereby air flows around the manifold ring and through the diffuser passageway mixing with fuel from the manifold ring and directed to a burn zone in the combustion chamber.
In a more specific embodiment of the present invention, the angle of the downstream portions of the diffuser inner and outer wall portions is selected to define the location of a burn zone in the combustion chamber.
Furthermore, in a yet more specific embodiment, the inlet may be offset relative to the inner and outer walls of the combustion casing in order to better locate the burn zone within the combustion chamber.
In a further embodiment of the present invention, a pair of annular air/fuel inlets is provided at the end of a combustion casing concentric with each other and with the inner and outer walls of the casing. The pair of annular air/fuel inlets includes an inner inlet adjacent the inner wall and an outer inlet adjacent the outer wall and an intermediate annular wall concentric with the inner and outer walls and located between the inner and outer inlets such that inner and outer combustion chambers are formed; each inner and outer air/fuel inlet including an inner and outer diffuser passageway respectively, wherein the outer passageway is formed between inner and intermediate diffuser portions of the outer and intermediate walls and wherein each outer and intermediate diffuser wall portion has an upstream and a downstream portion relative to the air flow; the inner passageway is formed between inner and intermediate diffuser portions of the inner and intermediate walls wherein each inner and intermediate diffuser wall portion has an upstream and a downstream portion relative to the air flow; the inner and outer diffuser passageways each include a converging cross-sectional section at the upstream portion of the diffuser wall portions and a diverging cross-section at the downstream portion of the diffuser wall portions and a throat is defined at the narrowest part of the passageway; and an inner and an outer concentric fuel manifold ring is provided upstream of each inner and outer diffuser passageway respectively whereby each inner and outer fuel manifold ring is located in axial alignment with the respective inner and outer diffuser passageway whereby the air flow flows around each manifold ring mixing with fuel from the respective inner and outer manifolds and through the respective inner and outer diffuser passageway and into the inner and outer combustion chamber respectively.